Membrane cleaning is a key step to the success of any ongoing membrane filtration process. Without regular cleaning the membranes become clogged with impurities and decrease in efficiency until they are eventually rendered inoperative. Different physical membrane cleaning strategies have been proposed and published. A summary of some typical methods is described below.
1. Scrubbing membranes with gas bubbles. This method was first published by Yamamoto et al. (Water Science Technology, Vol. 2, pages 43-54; 1989) and has been widely used in low-pressure filtration processes. The shear force created by gas bubbles removes fouling materials on the membrane surface, but does little to reduce the fouling within the membrane pores.2. Backwash or back pulsing method. This method uses a reversed flow of fluid (relative to the filtration flow direction) through the membrane pores to dislodge fouling materials therefrom. Either gas or liquid or a combination thereof can be used as a fluid in the reverse backwash.
In PCT Publication WO 03/059495, Bartels et al. describe a backwash technique where the hollow fibre membranes are pressurized with a gas on a feed side at a specified time during the backwash. They describe the periodic use of such backwashes to effectively remove fouling components from the hollow fibre membranes.
To carry out a liquid backwash, typically a liquid pump and a liquid holding tank are required. The pump delivers a permeate flow in a reverse direction to the normal filtration flow through the membrane pores to clean accumulated solids and impurities from the membranes pores.
In some pressurised filtration systems it has been found advantageous to use a gas pushed liquid backwash for the pressurized system which provides improved efficiency without compromising backwash performance. A gas pushed liquid backwash is generally achieved by sending compressed gas, typically air, to the lumen side of a hollow fibre membrane and pushing the filtrate remaining in the lumen at the end of a filtration cycle to shell or feed side of membrane wall. The backwash air pressure is typically set to more than 150 kPa. It is desirable to keep the air pressure required to a minimum in order to reduce the cost and energy requirements for production of the pressurised air.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.